1. Field of the Invention
The present invention relates to a high data rate write head and, more particularly, to a high linear density write head which is short, has double high pitch coils, a major heat sink for each coil and a high aspect ratio well-defined second pole tip.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A write head typically employs ferromagnetic first and second pole pieces which are capable of carrying flux signals for the purpose of writing magnetic impressions into a track on a magnetic medium, such as a rotating magnetic disk. Each of the first and second pole pieces has a yoke region which is located between a pole tip region and a back gap region. The pole tip region is located at the ABS and the back gap region is spaced from the pole tip region at a recessed location within the write head. At least one coil layer is embedded in an insulation stack which is located between the first and second pole pieces in the yoke region. A nonmagnetic write gap layer is located between the pole tip regions of the first and second pole pieces. The first and second pole pieces are magnetically connected at the back gap. Processing circuitry digitally energizes the write coil which induces flux into the first and second pole pieces so that flux signals bridge across the write gap at the ABS to write the aforementioned magnetic impressions or bits into the track of the rotating disk.
A write head is typically rated by its areal density which is a product of its linear bit density and its track width density. The linear bit density is the number of bits which can be written per linear inch along the track of a rotating magnetic disk and the track width density is the number of tracks that can be written per inch along a radius of the rotating magnetic disk. The linear bit density is quantified as bits per inch (BPI) and the track width density is quantified as tracks per inch (TPI). The linear bit density depends upon the thickness of the write gap layer. The thinner the write gap layer, the greater the number of bits the write head can write into the track of the rotating magnetic disk. The track width density is directly dependent upon the width of the second pole tip at the ABS. Efforts over the years to increase the areal density of write heads has resulted in computer storage capacities increasing from kilobytes to megabytes to gigabytes.
The first and second pole pieces, including the second pole tip, are typically fabricated by plating techniques. A strong-felt need is to fabricate second pole tips with submicron widths is limited by the resolution of the fabrication techniques. The second pole tip is typically fabricated by frame plating. Photoresist is employed to provide the frame and a seed layer is employed to provide a return path for the plating operation. A typical sequence of fabricating a second pole tip, as well as other components of the first and second pole pieces, is to sputter clean the wafer, sputter deposit a seed layer, such as nickel iron, on the wafer, spin a layer of photoresist on the wafer, light-image the photoresist layer through a mask to expose areas of the photoresist that are to be removed (assuming that the photoresist is a positive photoresist), develop the photoresist to remove the light-exposed areas to provide an opening in the photoresist at the pole tip region and then plate the second pole tip in the opening up to a desired height.
It is necessary that a second pole tip have sufficient volume at the ABS in order to conduct the required amount of flux for writing the signals into the magnetic disk. If the second pole tip is made thinner, it must be made higher in order to provide the necessary volume of magnetic material. Unfortunately, as the track width becomes narrower the resolution of the photoresist decreases. Resolution is quantified as aspect ratio which is the width of the second pole tip divided by the thickness of the photoresist. As the thickness of the photoresist increases the light penetration during the light-imaging step loses its columnation as it travels toward the bottom of the photoresist. The result is that the side walls of the photoresist frame are jagged which results in jagged side walls of the second pole tip.
The aforementioned problems are particularly manifested when the second pole tip and the yoke of the second pole piece are plated simultaneously in a common photoresist frame. In addition to loss of resolution with an increasing height of the second pole tip, there is also notching of the side walls of the photoresist frame, and consequently the second pole tip, due to reflection of light from a seed layer on the insulation stack immediately behind the pole tip region. One method to overcome this problem has been to employ a stitched “T”-shaped second pole piece which is fabricated by first making only the second pole tip portion with a photoresist frame and then subsequently making the yoke portion of the second pole piece with a second photoresist frame with the yoke portion being stitched (magnetically connected) to a stitch region at the top of the second pole tip. This type of second pole piece is referred to as a stitched “T” because the yoke portion extends laterally across the top of the pole tip portion, forming the configuration of a “T”. The yoke portion can be stitched across the entire top surface of the second pole tip in which case it is exposed at the ABS or it may be recessed from the ABS, as desired. Unfortunately, processing steps subsequent to the construction of the second pole tip decrease the height of the second pole tip and can seriously damage its side walls.
There is a continuing strong-felt need to increase the aforementioned areal density of the write head. When the linear bit density is increased the data rate of the head is increased and when a more narrow second pole tip is fabricated the track width density is increased. A goal has been to achieve a high data rate of 1 gigabit (Gb) per second. This means that more bits will have to be written by the write head per inch along the length of the track of a rotating magnetic disk. This, in turn, means that the coercivity of the material of the disk will have to be increased in order for the smaller bits along the track to produce a sufficient field so that the field can be sensed by the read head. Coercivity is the amount of applied field required to switch the orientation of the magnetization of the bit from one direction perpendicular to the plane of the disk to the opposite direction. Since coercivity of the magnetic material of a disk must be increased in order to increase the data rate the write head must be capable of producing a higher intensity magnetic field for switching the bits in the magnetic disk from one direction to the other. The different directions represent digital information which is processed to provide intelligent information. In order for the write head to write magnetic bits into the high coercivity disk the write signal of the head must exceed the coercivity of the disk. In order to implement this function the write head must be highly efficient.
One way to increase the data rate is to decrease the pitch of the coil layer. The pitch is the distance across one turn of the coil plus one space between the turn and the next turn. It would be desirable if the coil had a pitch of 1 μm or less. Unfortunately, when the data rate is increased with a low pitch coil the write head suffers from an increase in heat and increase in eddy currents between the first and second pole pieces. Eddy currents reduce the write current which, in turn, reduces the write signal across the gap of the write head. If the write current is increased in order to increase the write signal the amount of heat generated is increased. When the heat becomes excessive one or more layers of the write head can protrude at the ABS which destroys the write head. This protrusion is caused by high expansion of one or more photoresist layers in the insulation stack of the write head. One way to reduce the eddy currents in the write head is to reduce its length between the ABS and the back gap. This can be accomplished by employing two coil layers which are stacked one above the other. A shorter write head will also minimize flux leakage between the first and second pole pieces. Unfortunately, it has been difficult to fabricate a high pitch second coil layer above the first coil layer because of the profile of the first coil layer. It has been found that even with planarization above the first coil layer that the profile of the first coil layer impacts the fabrication of the second coil layer so that the pitch of the second coil layer has to be increased to prevent shorting between the turns of the second coil layer. Further, while the first coil layer has a heat sink in the layers therebelow, the second coil layer does not have a sufficient heat sink. These problems have seriously restricted the construction of dual coil write heads to achieve high data rates.
Another aspect in the construction of high areal density write heads is the track width density of the head. The track width density depends entirely upon the distance between the first and second side walls of the second pole tip. Frame plating is typically employed to fabricate the second pole tip. When the second pole tip is a separate component of the second pole piece, which is fabricated before the fabrication of the yoke portion of the second pole piece, the first and second side surfaces of the second pole tip can be well-defined. Unfortunately, after forming the second pole tip the first and second side surfaces are seriously altered by subsequent processing steps involved in the fabrication of the second coil layer. Accordingly, in order to achieve high track width density the first and second side surfaces of the second pole tip must be protected after fabrication of the second pole tip.